In this specification reference is made to Low Temperature Fischer-Tropsch (LTFT) process. This LTFT process is a well known process in which carbon monoxide and hydrogen are reacted over an iron, cobalt, nickel or ruthenium containing catalyst to produce a mixture of straight and branched chain hydrocarbons ranging from methane to waxes and smaller amounts of oxygenates. This hydrocarbon synthesis process is based on the Fischer-Tropsch reaction:2H2+CO→˜[CH2]˜+H2Owhere ˜[CH2]˜ is the basic building block of the hydrocarbon product molecules.
The LTFT process is used industrially to convert synthesis gas, which may be derived from coal, natural gas, biomass or heavy oil streams, into hydrocarbons ranging from methane to species with molecular masses above 1400. While the term Gas-to-Liquid (GTL) process refers to schemes based on natural gas, i.e. methane, to obtain the synthesis gas, the quality of the synthetic products is essentially the same once the synthesis conditions and the product work-up are defined.
While the main products are linear paraffinic materials, other species such as branched paraffins, olefins and oxygenated components may form part of the product slate. The exact product slate depends on reactor configuration, operating conditions and the catalyst that is employed, as is evident from articles such as Catal. Rev.-Sci. Eng., 23 (1&2), 265-278 (1981) or Hydroc. Proc. 8, 121-124 (1982).
Preferred reactors for the production of heavier hydrocarbons are slurry bed or tubular fixed bed reactors, while operating conditions are preferably in the range of 160-280° C., in some cases in the 210-260° C. range, and 18-50 bar, in some cases preferably between 20-30 bar.
The catalyst may comprise active metals such as iron, cobalt, nickel or ruthenium. While each catalyst will give its own unique product slate, in all cases the product slate contains some waxy, highly paraffinic material which needs to be further upgraded into usable products. The LTFT products can be hydroconverted into a range of final products, such as middle distillates, naphtha, solvents, lube oil bases, etc. Such hydroconversion, which usually consists of a range of processes such as hydrocracking, hydrotreatment and distillation, can be termed a LTFT Products Work-up process. Typically the process is normally configured in such a way that only two liquid products are transferred to storage. In most instances a small amount of light hydrocarbons containing up to four carbon atoms is also co-produced. The typical quality of the LTFT liquid products is presented in Table 1.
TABLE 1Typical Quality of the LTFT ProductsLTFT NaphthaLTFT DieselDensity, kg/l (20° C.)0.6850.765DistillationIBP, ° C.54151T10, ° C.81182T50, ° C.101249T90, ° C.120317FBP, ° C.131334Composition, % wtn-paraffins59.031.9iso-paraffins38.267.1NaphthenicsNDNDAromatics0.3NDOlefins2.5NDOxygenatesNDNDIso:Normal Paraffin ratio0.652.10
The applicant has identified a need to utilise LTFT fuel, including GTL fuel, directly, without blending with cracked stocks, as a fuel that will be interchangeable with conventional diesel fuels.
Semi-synthetic aviation fuel was approved in 1999 under British Aviation Turbine Fuel Defence Standard 91-91 (DEF STAN 91-91) specifications.
A need has thus been identified for a synthetic based fuel which meets or exceeds the above standards and which permits use of LTFT products, including GTL products, or components thereof in the aviation industry as fuels and/or as blend stocks for fuels.